1. Field of the Invention
The present invention relates to an airflow meter of a so-called heat (or hot) wire type with an improved exothermic resistance element and a manufacturing method of the improved resistance element.
2. Description of the Related Art
There has been already known an airflow meter using a resistance element made functioning as an exothermic resistor, in which a current flowing through the resistance element is adjusted so as to maintain the temperature of the element at a predetermined value and the temperature drop in the element, which is caused by air flowing in contact therewith, is detected as change in the current flowing therethrough, whereby the rate of the airflow can be measured. Such an airflow meter is known as a heat wire type airflow meter, which is now widely employed for the purpose of measuring the airflow rate of an intake air of an internal combustion engine.
Also, there have been already known various sorts of resistance elements used in the heat wire type airflow meter. As one of them, there is a resistance element, in which a resistance wire such as platinum, nickel, platinum-cobalt alloy and so on is wound on a bobbin of sufficiently sintered ceramics to form a heat wire coil functioning as an exothermic resistor (JP-A-54/145166(1979)). Since such a heat wire is very fine, the ceramic bobbin gives the sufficient mechanical strength to the heat wire coil and, hence, to the resistance element as a whole.
However, although the aforesaid resistance element with the ceramic bobbin has the sufficient mechanical strength, it is accompanied with the following defect. Namely, a part of heat generated by the heat wire coil is consumed to heat the ceramic bobbin, and the heat transferred to the bobbin in turn escapes outside through support members coupled to the bobbin. This results in the reduction in the measuring accuracy. In addition, the response time of the airflow meter to change in the rate of airflow to be measured becomes worse due to the heat capacity of the bobbin.
To improve this defect, one of the inventors of the present invention has already proposed a heat wire type airflow meter, which has an improved resistance element without any bobbin, i.e., a bobbinless resistance element (JP-A-62/83622 (published Apr. 17, 1987)). According to the prior art described above, an exothermic resistance wire is wound in the coil form on a core wire, and after the coiled resistance wire is coated and solidified by glass material, the core wire is removed by acid, whereby a bobbinless resistance element is obtained and an airflow meter, which is free from the influence of the heat capacity of the ceramic bobbin, can be realized.
It has been found, however, that the prior art has the following problem, especially when molybdenum is used as a core wire for winding a resistance wire. Namely, upon sintering of a coating layer of glass, glass is crystallized under the influence of the molybdenum core wire, with the result that the evenness of the outer surface of a finished resistance element is lost. The roughness of the outer surface of a resistance element causes the disturbance in the airflow to be measured and the accumulation of dust included in air thereon. Those facts exert a great influence upon the measuring accuracy of a heat wire type airflow meter.